Process control systems require the accurate measurement of process variables. Typically, a primary element senses the value of a process variable and a transmitter develops an output having a value that varies as a function of the process variable. For example, a level transmitter includes a primary element for sensing level and a circuit for developing an electrical signal proportional to sensed level.
An electrical transmitter must be connected to an electrical power source to operate. One form of such a transmitter, known as a four-wire transmitter, includes two terminals for connection to a power source and two terminals for carrying an output signal proportional to the process variable. This signal can be used as an input to a controller or for purposes of indication. Because the instrument is connected directly to a power source independent from the output signal, power consumption is a less critical factor in design and use of the same.
The use of a four-wire transmitter, as discussed above, requires the use of four conductors between the transmitter and related loop control and power components. Where transmitters are remotely located, such a requirement can be undesirable owing to the significant cost of cabling. To avoid this problem, instrument manufacturers have strived to develop devices known as two-wire, or loop powered, transmitters. A two-wire transmitter includes two terminals connected to a remote power supply. The transmitter loop current, drawn from the power supply, is proportional to the process variable. A typical instrument operates off of a 24 volt DC power supply and varies the signal current in the loop between four and twenty milliamps (mA) DC. Thus, the instrument must operate with current less than four milliamps.
While low power circuits are continuously developed, there are other increasing demands placed on performance capabilities of the process control instruments. For example, with a guided wave radar level measurement device, the instrument's performance is enhanced by more powerful digital signal processing techniques driven by a microprocessor. In addition to the microprocessor, there are several other circuits, such as the radar transceiver, which requires electric power. To be successful, the instrument must use optimum processing capability and speed. This means making maximum power from the loop available to the electronics, and using it efficiently.
More recently, the loop powered instruments have utilized digital communications. In normal operation, the instrument must allow for 4 mA to 20 mA loop currents while still communicating digital signals via modulation of the loop current. Loop currents as low as 3.6 mA or as high as 22 mA are allowed when the transmitter detects a fault condition. This means that the power available at the input to the switching power supply, which powers the entire transmitter, will be based on input voltage to the switching power supply and the nominal loop current. However, the actual power available will also be based on the efficiency of the switching power supply. In addition, it is necessary to maintain high input impedance for digital communications.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.